In vivo 3D OCT image, which shows the mouse oviduct with preimplantation embryos (red spheres) inside. Credit:
Huan Han and Shang Wang, Stevens Institute of Technology
The fallopian tube, also known as the oviduct, is responsible for several critical processes that lead to pregnancy.
The tubular structure transports sperm and eggs, hosts fertilisation and shepherds preimplantation embryos toward the uterus as they develop.
Shang Wang, an assistant professor in the department of biomedical engineering at Stevens Institute of Technology in the US, says most of the oviduct’s functions haven’t been observed in their natural environment.
“We don’t yet know what biological mechanisms ensure they work properly,” he says. “This lack of information is a key reason why the causes of tubal ectopic pregnancy and oviduct-related infertility remain largely unknown.”
“Little is known in this critical area due to the technical difficulty in studying it.”
Wang and colleagues have used “optical coherence tomography” (OCT) to peer inside the abdomens of female mice. The approach opens what Wang describes as “a unique window into embryo movement and the early stage of embryo development inside the fallopian tube”.
The team performed surgery on 12 female mice, implanting a small window made of glass and resin to allow them to bypass the skin and muscle and directly visualise the fallopian tubes.
The mice were kept under anaesthesia for the imaging process.
They shone through the window near-infrared light which can penetrate tissues to depths of several hundred micrometres. The backscattered light is then measured to reconstruct the depth of the sample to produce a detailed 3D image.
Doing this over time allowed the researchers to capture both the oviduct dynamics and the embryo movement within the fallopian tube.
“OCT was ideal for this study because it provided label-free 3D imaging at a scale that resolved structural details throughout the oviduct’s inner space, while capturing images fast enough to visualise tissue and cell dynamics,” says Huan Han, a doctoral student in Wang’s laboratory.
(A) In vivo imaging setup with a clamp stabilising the window implanted on the right dorsal side of the mouse. (B) In vivo bright-field image of the oviduct as well as the ovary and a portion of the uterus through the window. (C) In vivo 3D OCT image of the oviduct showing its 3D morphology and structure through the window. Scale bars are 500 µm. Credit: Han et al 2025, Biomedical Optics Express, DOI:10.1364/BOE.565065
This revealed a previously unknown “leaky peristaltic pump” mechanism drives the forward-backward movement of the embryo towards the uterus.
Muscular contraction waves originated in the region of the fallopian tube where fertilisation occurs, the ampula, and propagated through the isthmus – the region closer to the uterus where embryos develop during preimplantation.
This contraction wave pushed fluid and embryo forward, while muscular relaxation at earlier contraction sites pulled it back through suction. Constriction of the fallopian tube lining at oviduct turning points could also stop this backward embryo movement at times.
Together this results in “bidirectional” movement of the embryo – like taking 2 steps forward and 1 step back – and slow net movement toward the uterus.
This seemingly inefficient process could be just what the embryo needs to develop properly.
“During the transport process in the isthmus, embryos undergo their critical preimplantation development and become ready for their implantation in the uterus,” the authors write.
“In terms of delivering embryos to the uterus, the uncovered leaky peristaltic pumping process with the bidirectional embryo movement is highly inefficient; however, this particular way of pumping and transport provides time for the developmental process, enabling successful pregnancy.
“In addition … the pumping dynamics with the oscillation of embryos is thought to play an important role in generating mechanical cues that support or even regulate cellular activities during the preimplantation development, which we will study in our future work.”
“Now that we understand the normal process of how the embryos are transported, it is possible to investigate the abnormal processes underlying related disorders and diseases,” says Wang.
“This research is just the beginning of uncovering how the oviduct supports pregnancy and early embryo development, which could ultimately lead to better strategies for clinical care of ectopic pregnancy and certain forms of infertility,” adds Han.
The findings are published in the journal Biomedical Optics Express.